Plants are constantly exposed to a variety of biotic stress such as pathogen infection or insect herbivory and abiotic stresses such as high or low temperatures, drought and salinity. To survive these challenges, plants have developed elaborate mechanisms to detect external signals and manifest adaptive responses with proper physiological and morphological changes (Bohnert et al., 1995). Detection of extracellular stimuli and subsequent activation of defense responses requires a complex interplay of signaling cascades in which reversible protein phosphorylation plays a central role (Yang et al., 1997).
Increasing evidence has shown that the intracellular signaling module, the mitogen-activated protein kinase (MAPK) cascade plays an important role in plant signal transduction related to biotic and abiotic stresses. This phosphorylation cascade typically consists of three functionally interlinked protein kinases: a MAP kinase kinase kinase (MAPKKK), a MAP kinase kinase (MAPKK) and a MAP kinase (MAP kinase). In this phosphorylation module, a MAPKKK phosphorylates and activates a particular MAPKK which in turn phosphorylates and activates a MAPK. Activated MAPK is often imported into the nucleus where it phosphorylates and activates specific downstream signaling components such as transcription factors (Khokhlatchev et al., 1998).
Activation of MAPKs has been observed in plants exposed to pathogens (Suzuki and Shinshi, 1995; Adam et al., 1997; Ligternik et al., 1997; Zhang and Klessig, 1997, 1998b; He et al., 1999), cold (Jonak et al, 11996), and wounding (Seo et al., 1995; Usami et al., 1995; Bogre et al., 1997; Zhang and Kiessig, 1998a; Seo et al., 1999; He et al., 1999). Plant MAPKs can also be activated by fungal elicitors (Suzuki and Shinshi, 1995), salicylic acid (Zhang and Klessig, 1997), jasmonic acid (Seo et al., 1999), and abscisic acid (Knetsch et al., 1996; Burnett et al., 2000; Heimorvaara-Dijkstra et al., 2000). Although, considerable progress has been made in cloning and characterization of plant MAPKKs (Morris et al., 1997; Ichimura et al., 1998a; Hackett et al., 1998; Hardin and Wolniak, 1998; Kiegerl et al., 2000 Yang et al., 2001) and MAPKKKs (Ichimura et al., 1998b; Kovtun et al., 2000; Frye et al., 2001), detailed steps of MAP kinase cascades have yet to be elucidated in any plant species. Upstream MAPKKs for dicot MAPKs such as NtMEK2 for SIPK/WIPK in tobacco (Yang et., 2001), AtMEK1 for AtMPK4 in Arabidopsis (Huang et al., 2000), and SIMKK for SIMK in alfalfa (Kiegerl et al., 2000) have been determined. The complete MAP kinase cascade (EKK1), MKK4/MKK5 and MPK3/MPK6 together with its upstream receptor kinase FLS2 and downstream WRKY22/WRKY29 transcription factors was characterized in Arabidopsis (Asai et al., 2002). These findings suggest that MAPKs are important signaling components in plant defense responses and that the cascade of a “three-kinase module” is a general mechanism of defense signal transduction among eukaryotic organisms (Ligterink and Hirt 2000).
Recently, protein kinases possessing close sequence similarity to the mammalian MAPKs have been identified in plants (Stone and Walker, 1995; Hirt, 1997; Mizoguchi et al, 1997; Tena et al., 2001; Ahang and Klessig, 2001; Tchimura et al., 2002). However, despite this progress, most characterized plant MAPKs were isolated from dicot model species such as Arabidopsis and tobacco and our understanding of the role of MAPK cascades in stress response remains rather limited. Moreover, very few MAPKs have been identified and characterized in economically important monocot species such as rice, maize, wheat or barley. Rice is not only principal food crop for over half of the world's population, but also an excellent model for cereal crops because of its relatively small genome, extensive genetic mapping data, relatively easy transformation and synteny with other cereal genomes. A MAP kinase, OsBWMK1 found in rice leaf was determined to be activated by blast fungus infection and wounding (He et al., 1999) and a stress-responsive rice MAP kinase gene (variously named OsMAPK5, OsMSRMK2, OsMAPK2, OsMAP1 or OsBIMK1) was identified and shown to be induced at the mRNA level by multiple biotic and abiotic stresses (Xiong et al., 2001; Agrawal et al., 2002; Huang et al., 2002: Wen et al., 2002; Song et al., 2002). Plant MAPKs are encoded by a multigene family and play a pivot role in plant growth and development as well as biotic and abiotic stress responses. As a result, functional genomic analysis of the entire MAPK gene family in rice should significantly enhanced our understanding of the MAPK-mediated signaling network in monocots and its effects on agronomically important traits such as yield, quality, pest resistance and abiotic stress tolerance.
Discussion or citation of a reference herein shall not be construed as an admission that such reference is prior art to the present invention.